1. Field of the Invention
The present invention relates to electric double layer capacitors and, in particular, to such capacitors that employ activated carbon in their polarizable electrodes.
2. Description of the Related Art
The electric double layer capacitor is a capacitor that utilizes a phenomenon wherein electric energy is stored, in the form of electric charge, in an electric double layer formed at an interface between the polarizable electrode and an electrolyte.
The electric double layer capacitor has a large capacitance for its size and has a wide application as a back-up power source for electronic devices such as microprocessors, electronic memory devices, electronic timers and the like.
Representative arrangements of the conventional electric double layer capacitors are those of cylindrical type as shown in, for instance, the U.S. Pat. No. 5,150,283. In a typical arrangement, a sheet-like current collector of one polarity, whose surfaces are formed into the polarizable electrode composed of activated carbon and a binder, and another sheet-like current collector of the other polarity are wound into a roll by interlaying a separator therebetween. The wound roll is then impregnated with the electrolyte solution, and thereafter housed in a cylindrical container whose open end is sealed with a rubber packing. Lead wires connected to the respective current collectors are drawn out of the container by piercing through the rubber packing.
The above-mentioned cylindrical electric double layer capacitor has a high capacitance density, i.e., a large capacitance for its unit volume. However, the cylindrical capacitor sometimes has an insufficient volumetric efficiency when it is installed in certain electronic appliances. When arranging a plurality of the cylindrical capacitors in serial or parallel connection for securing a large capacitance or a high working voltage, a plurality of dead spaces are inevitably created between the laterally aligned capacitors, thereby resulting a large total dead space and considerably deteriorating volumetric efficiency.
In addition, in the above-discussed conventional electric double layer capacitor, the lead wires are electrically connected to only small spots of the current collectors in a long belt shape. Therefore, the conventional electric double layer capacitor has a disadvantage that an impedance at the lead wire part is large. Usually, the electric double layer capacitor comprising activated carbon is employed where a large current such as from 100 A to 1,000 , is required for a time period of lasting from several seconds to several tens seconds. In that case, the lead wire parts are required to have a low impedance. In contrast to this, in case of an aluminum electrolytic capacitor, the time during which a large current is flown is limited to only a range of several micro seconds to several milliseconds, and hence, the decreases in the impedance at its lead wire parts are not so necessary as in the case of the electric double layer capacitor.
In order to decrease the impedance in the lead wire parts, the number of the lead wires must be increased in the conventional capacitors of the above-mentioned structure, but increasing the number of the lead wires is difficult to realize due to the structure of such capacitors.